This invention relates to semiconductor integrated circuit devices, and more particularly to output drivers for CMOS integrated circuits.
In my U.S. Pat. No. 4,963,766, assigned to Digital Equipment Corporation, a CMOS driver circuit is shown which is of the type used as an output buffer in a single-chip microprocessor or memory circuit. A driver of this type is used to convert a logic voltage within the chip to a higher-level signal for driving a bus on a printed circuit board or other connection between chips. The circuit of Pat. No. 4,963,766 is adapted to be powered by a low-voltage supply, e.g., 3.3 v., but yet is able to withstand higher voltages imposed on its output node. A P-channel pull-up transistor is employed, and the N-well of this pull-up transistor is connected to a higher-level voltage supply, 5.0-volts.
As CMOS feature sizes continue to shrink, the resulting device speed has continually improved. Faster rise and fall times incident to these faster device speeds results in the etched-circuit conductors of a module changing from an RC-dominated environment to an LC or transmission line environment. An etch-conductor with its grounded plane, usually found in a circuit board, forms a transmission line at the frequencies present in logic voltages having fast rise and fall times. A transmission line environment requires that the driver, etched-conductor, and loading be carefully analyzed to ensure that transmission line effects such as overshoot, undershoot, and ringing do not impact data transfers.
One prior solution to the problems of the transmission line environment is to use series-source terminations on the chip. That is, a resistor is placed in series with the signal source, so the resistor appears in series between the output pad and the driver transistor. The resistor should match the impedance Z.sub.O of the transmission line appearing at the chip output. However, a series source termination is usually optimized for etched-conductor impedance, loading, and driver position on the etch (end or middle). Both the resistor (a diffusion) and the NMOS pull-down transistor are subject to process variations in manufacture and temperature variations in operation. It is thus desirable to find a solution that is more independent of these parameters.